1. Field of the Invention
The present invention generally relates to an apparatus having a fluid-filled cavity between two housing elements fixed relative to one another. The size of this cavity is adjustable to compensate for volumetric contraction and expansion of the fluid. More particularly, the present invention is directed to an assembly comprising a metal pad, an acoustic transducer and such a fluid-filled cavity capable of compensating for pressure and temperature changes experienced when using the assembly in a tool in a deep oil or gas well.
2. Description of the Background
Many measurements using acoustic transducers are made in the oil and gas industry in open and cased wells. Logging tools, in particular sondes from which these acoustic transducers are disposed, are passed through a borehole or a completed well to provide valuable information concerning the completed well and the formation through which the borehole has been drilled. For example, acoustic logging is often used to determine the quality of the cement bond between a casing and the surrounding formation in a completed well.
In operation, these transducers are subjected to extremes of temperature and pressure. The temperature and pressure at a downhole location is generally proportional to the depth of the location. Both temperature and pressure increase with increasing depth of the well. For example, these transducers are subjected to temperatures as high as about 350.degree. F. and pressures as high as about 20,000 psi in boreholes in typical drilling operations. However, temperatures as high as about 500.degree. F. and pressures as high as about 25,000 psi may be encountered in deep wells drilled to depths of about 25,000 feet.
Because the surface of the borehole or the surface of the inside of the casing is rough, it is desirable to separate the acoustic transducer from the borehole by a metal pad which will absorb the wear and abrasion of use. The metal pad is generally curved on one side for contact with the borehole or the inside wall of the casing and is typically provided with a recess on the other side into which the acoustic transducer is secured To prevent the entrance of abrasive drilling fluids between the transducer and the pad and in order to provide improved acoustic transmission between the acoustic transducer and the pad, the recess is typically filled with a protective oil prior to installation of the transducer into the recess of the pad, resulting in a thin layer of oil being trapped at the contact surface between the transducer and pad and in a small reservoir surrounding the contact surface
This trapped oil expands and contracts in response to changes in the pressure and temperature in accord with its known coefficients of expansion. For example, the volume occupied by the trapped oil increases at the high temperatures encountered downhole in oil and gas wells. In typical downhole logging operations, the volume occupied by this protective oil increases by approximately 10-12 percent as the temperature increases from ambient to about 350.degree. F. In deep wells this expansion may be as much as about 15-20 percent at temperatures of about 500.degree. F. The drastic increases in pressure encountered downhole also affect changes in the volume of these protective oils, although the effect of pressure changes on these liquids is not so significant as the effect of temperature changes.
However, early designs for acoustic transducers used in logging tools downhole made no provision for these changes in volume of the trapped oil. Although these changes were taken into account in some later systems by inclusion of various devices, including mechanical bellows, pistons or diaphragms, a simple and satisfactory compensation system was not developed. Each of the prior devices suffers from one or more major drawbacks. Some prior transducer assemblies did not address or compensate for the problem created by the changing volume of the trapped oil. Other assemblies which included devices which addressed this problem and attempted to compensate for it were complex and expensive to construct and maintain
Accordingly, there has been a long felt but unfulfilled need within the oil and gas industry for a simple, effective, economical and reliable apparatus for compensating for this expected change in fluid volume